1. Field Of The Invention
Continuing emphasis on environmental quality and recent new emphasis on air quality in particular have resulted in increasingly stringent regulatory control of industrial emissions. One technique which has proven highly effective in controlling air pollution is the removal of undesired particulate matter from a gas stream by electrostatic precipitation.
An electrostatic precipitator is an air pollution control device designated to electrically charge and collect particulates generated from industrial processes such as those occurring in cement plants, pulp and paper mills and utilities. Particulate-laden gas flows through the precipitator where the particles acquire a charge. These charged particles are attracted to, and collected by, oppositely-charged metal plates. The cleaned process gas may then be further processed or safely discharged to the atmosphere.
The electrostatic precipitation process involves several complicated, interrelated physical mechanisms: The creation of a nonuniform electric field and ionic current in a corona discharge; the ionic and electronic charging of particles moving in combined electro- and hydrodynamic fields; and the turbulent transport of charged particles to a collection surface. Because of this, many practical considerations can act to reduce collection efficiency.
To maximize the particulate collection, a precipitator should operate at the highest practical usable energy level, increasing both the particle charge and collection capabilities of the system. At the same time, there is an energy level above which arcing or "sparking", a temporary short which creates a conductive gas path, occurs in the system. Maximizing the efficiency of an electrostatic precipitator requires operating the system at the highest possible usable energy level.
Ideally, the electrostatic precipitator should operate constantly at its point of greatest efficiency. Unfortunately, conditions under which an electrostatic precipitator operates, such as temperature, combustion rate, and the chemical composition of the particles being collected, change constantly. This complicates calculating parameters critical to a precipitator's operation.
2. Description Of The Prior Art
This invention relates to electrostatic precipitators in general and specifically to precipitator power supplies. Prior art precipitator power supplies have used either saturable core reactors or silicon-controlled rectifiers (SCRs) paired with a fixed-value current-limiting reactor (CLR). This invention relates to an improvement of the CLR.
Prior art CLRs have an inductance of fixed value with several taps for selecting other values. The number of taps available is limited, typically to three. Adjusting the inductance of the CLR requires that the precipitator field section be powered down and taps manually changed.
A CLR of the correct value contributes to protecting the precipitator power supply from the destructive effects of arcing or spark currents and ensures greater electrical and particulate collection efficiencies.
Prior art devices useful for voltage and current control of power supplies have been disclosed in various patents including U.S. Pat. No. 1,372,653 issued Mar. 22, 1921 to F. Dessauer on an Electrical Transformer System; U.S. Pat. No. 1,702,771 issued Feb. 19, 1929 to Y. Groeneveld on an Amplifying Transformer; U.S. Pat. No. 1,732,715 issued Oct. 22, 1929 to F. Dessauer et al on an Electromagnetic Induction Apparatus; U.S. Pat. No. 1,896,480 issued Feb. 7, 1933 to A. Christopher on a Balanced Inductance Device; U.S. Pat. No. 2,878,455 issued Mar. 17, 1959 to C. Lamberton et al on a Three Winding Transformer; U.S. Pat. No. 3,483,499 issued Dec. 9, 1969 to L. Lugten on an Inductive Device; U.S. Pat. No. 4,020,438 issued Apr. 26, 1977 to A. Manimalethu on an Autotransformer With Series And Tertiary Windings Having Same Polarity Impedance; U.S. Pat. No. 4,513,274 issued Apr. 23, 1985 to M. Halder on a Current Transformer For Measuring Instruments; U.S. Pat. No. 4,590,453 issued May 20, 1986 to A. Weissman on an Autotransformer With Common Winding Having Oppositely Wound Sections; U.S. Pat. No. 4,916,425 issued Apr. 10, 1990 to N. Zabar on an Electromagnetic Device and U.S. Pat. No. 4,973,930 issued Nov. 27, 1990 to U. Mai et al on a Twin Coil.
An alternative to the silicon-controlled rectifiers paired with a fixed-value current limiting reactor is a saturable core reactor. The saturable core reactor (or saturable reactor) was originally developed in Germany and was used in the United States extensively from 1945 forward. The principal application has been to control the power applied to heating elements. Saturable reactors are electrically and mechanically rugged. In recent years, their functions have been largely taken over by silicon-controlled rectifiers; as a consequence, the saturable reactor has been relegated to obscurity.